Methods and apparatus for acoustic fiber fractionation

ABSTRACT

Methods and apparatus for acoustic fiber fractionation using a plane ultrasonic wave field interacting with water suspended fibers circulating in a channel flow using acoustic radiation forces to separate fibers into two or more fractions based on fiber radius, with applications of the separation concept in the pulp and paper industry. The continuous process relies on the use of a wall-mounted, rectangular cross-section piezoelectric ceramic transducer to selectively deflect flowing fibers as they penetrate the ultrasonic field. The described embodiment uses a transducer frequency of approximately 150 kHz. Depending upon the amount of dissolved gas in water, separation is obtained using a standing or a traveling wave field.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the separation of wood pulpfibers into various fractions such as long and short fibers forefficiencies in the paper making process, especially concerning papermill effluents containing pulp slurry recyclable through separation oflarge radius fibers and small radius fibers into a concentrated paperfiber stock of the relatively larger fibers for paper making and a cleanstream of the relatively smaller fibers from a stream of dilute fibersuspension. More particularly the invention relates to an acoustic cell,an acoustic fractionator and methods of separating dilute suspensions offibers for fiber fractionation with acoustic separation using acousticwave fields. Such acoustic wave fields induce deflections of the fiberspenetrating an ultrasonic field imposing agglomeration and reorientationon the fibers of fiber suspensions to separate the fibers into two ormore fractions according to the relative sizes of the fibers.

2. Description of the Related Art

Separation of virgin or reclaimed wood pulp fibers into two or morefractions which are relatively enriched in longer or shorter fibers isan important step of the paper-making process, because it allows for theefficient use of fiber properties. Fiber fractionation allows anoptimized use of raw materials, increases production versatility, andcontributes to waste and energy consumption reduction. Typical examplesare: optimization of multi-layered products by placing fractions wherethey are most needed in the sheet; energy savings by restricting pulprefining to the long-fiber fraction; separation of valuable fractionfrom waste.

Various technologies have been devised during the past forty years tofractionate wood pulp fibers. See, e.g., P. Seifert and K. L. Long,"Fiber fractionation--Methods and Applications," Tappi J., Vol. 57(10)pp. 69-72 (1974); and T. F. Skaar, "Fractionation: Equipment,Applications and Process Control," Proc. of the Pulping Conf., TappiPress, pp. 211-216 (1984). Pressure screen systems, which fractionatefibers based on fiber length, are generally perceived as the mostsuccessful technology on a commercial stand-point. See, H. Ibrahim,"Fiber Fractionation--An appropriate Technology for Upgrading RecycledFibers and Saving Energy," Proc. Eucepa Symp. "Recycling of Fibres andFillers in Pulp and Paper Industry" Ljubljana pp. 539-569 (Oct. 23-27,1989).

In pressure screen systems, pulp slurries circulate between a stationarycylinder-shape screen and an external rotor. Pressure conditions betweenthe screen and the rotor are such that the short fibers pass through thescreen; long fibers are retained on the screen. The separationefficiency depends upon pulp furnish, use of perforated or slottedscreens, pulp consistency, and input and output flow rates. Thetechnology has high throughput. However, pressure screens have limitedon-line adaptability to variable pulp furnishes, and they also havelimited on-line adaptability for variable product requirements. Also,separation efficiency is not always satisfactory and multi-stage systemsare used to meet separation goals. These are drawbacks when oneconsiders the ever increasing use of reclaimed fibers from mixed grades.Moreover, pressure screen systems cannot fulfill fractionationrequirements based on fiber radius, wall thickness, or coarseness,especially the separation of springwood and summerwood fibers or theseparation of softwood and hardwood fibers, or in the separation ofshives from the fibers.

It would be desirable to employ a versatile mechanism facilitatingon-line adaptability to variable pulp furnishes and variable productrequirements such as fiber fractionation with acoustic separation usingultrasonic wave fields, but very little knowledge has been available onthe interaction of a sound field with fibers and more generally withprolate spheroids and cylindrical particles. Awatani was the first tocalculate the acoustic force on a prolate spheroid, J. Awatani, "On theAcoustic Radiation Pressure on a Prolate Spheroid," Memo. Inst. Sci.Ind. Res., Osaka U., Vol. 10, pp. 51-65 (1953), and a rigid circularcylinder, J. Awantani, "Study on Acoustic Radiation Pressure (IV),Radiation Pressure on a Cylinder," Memo. Inst. Sci. Ind. Res., Osaka U.,Vol. 12, pp. 95-102 (1955), in plane traveling and standing wave fields.He found that the force on a prolate spheroid, whose axis of symmetry isperpendicular to the sound field direction (stable orientation), islarger than that on a disc or a sphere which has the same projectivearea. More recently, Zhuck and Wu et al. reported independentderivations of the acoustic force on a rigid cylinder at stableorientation, in plane traveling and standing wave fields, respectively.See, A. P. Zhuck, "Radiation Force Acting on a Cylinder Particle in aSound Field," Sov. Appl. Mech., Vol. 22, pp. 689-693 (1987); and J. WuG. Du, S. S. Work and D. M. Warshaw, "Acoustic Radiation Pressure on aRigid Cylinder: An Analytical Theory and Experiments," J. Acoust. Soc.Am., Vol. 87, pp. 581-586 (1990).

SUMMARY OF THE INVENTION

The present invention teaches the use of a novel concept for thecontinuous separation of fibers into two or more fractions. Thedescribed embodiments are based upon the use of a plane ultrasonic wavefield to induce lateral deflections of moving fiber suspensions in achannel flow, and therefore, separate fibers accordingly to thedeflection mechanism. Since the acoustic radiation force acting on thefibers is primarily a function of the fiber diameter or radius (i.e.,fiber width), large radius fibers are more deflected than small radiusfibers. In practice, fibers interacting with the ultrasonic field can becollected by at least two discharge streams, one concentrated stream forthe deflected fibers (large radius fibers) and one clean stream for theweakly deflected or undeflected fibers (small radius fibers and/or fiberdebris).

It is an object of the present invention to provide methods andapparatus to overcome the problems and disadvantages of the prior artfractionation techniques.

It is another object of the present invention to separate paper fibersslurry into fractions concentrated and clean fiber streams forefficiencies in the paper making process with acoustic separation usingultrasonic wave fields to induce deflections of the fibers.

It is a further object of the present invention to provide an acousticcell for subjecting dilute suspensions of fibers to deflection forces,the fibers being of differing relative sizes.

It is a still further object of the present invention to provide anacoustic fractionator for separating dilute suspensions of fibers intoplural fractions according to the relative fiber sizes of differingfibers.

It is yet another object of the present invention to provide a method ofseparating dilute suspensions of fibers into plural fractions accordingto the relative fiber sizes of differing fibers.

Briefly, the present invention relates to methods and apparatus foracoustic fiber fractionation using an acoustic cell for subjectingdilute suspensions of fibers to deflection forces, the fibers being ofdiffering relative sizes, and an acoustic fractionator for separatingdilute suspensions of fibers into plural fractions according to therelative fiber sizes of differing fibers. The acoustic cell provides atubular element for directing moving fiber suspensions in a channelflow, an input for receiving time varying signals, and a transducer forgenerating an ultrasonic field responsive to the time varying signals toinduce deflections of the fibers imposing agglomeration andreorientation on the fibers of the fiber suspensions to separate thefibers into at least two fractions according to the relative sizes ofthe fibers.

The acoustic fractionator and a method of separating dilute suspensionsof fibers into plural fractions according to the relative fiber sizes ofdiffering fibers convey a feed stream of dilute fiber suspension,transfer the feed stream through an acoustic cell for directing movingfiber suspensions in a channel flow, generate an ultrasonic field uponthe channel flow to induce deflections of the fibers in the acousticcell imposing agglomeration and reorientation on the fibers of the fibersuspensions to separate the fibers into at least two fractions accordingto the relative sizes of the fibers and then divide the fibersuspensions at the down stream side of the acoustic cell to separate thefibers of the fiber suspensions into at least two fractionscorresponding to at least two outlet streams of fibers according to therelative sizes of the fibers. The acoustic cell may have an absorberplate coupled to a wall section opposite the transducer for producing atraveling wave field, or in another embodiment, the acoustic cell mayhave a reflector plate coupled to the wall section opposite saidtransducer for producing a standing wave field.

The appended claims set forth the features of the present invention withparticularity. The invention, together with its objects and advantages,may be best understood from the following detailed description taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an experimental setup for studying the acoustic radiationeffects on moving fiber suspensions in accordance with the invention;

FIG. 2 shows a schematic block diagram thereof;

FIG. 3 shows the acoustic cell for the case of a one-dimensional,unbalanced acoustic resonator embodiment wherein the acoustic cellutilizes a reflector plate coupled to the wall section opposite saidtransducer for producing a standing wave field;

FIG. 4A illustrates a plane traveling wave acoustic field in theacoustic cell embodiment for producing a traveling wave field therein;

FIG. 4B illustrates a plane standing wave acoustic field in anotheracoustic cell embodiment for producing a standing wave field therein;

FIGS. 5A through 5D are photographs which illustrate deflections of thefibers penetrating an ultrasonic field imposing agglomeration andreorientation on the fibers of the fiber suspensions to separate thefibers into plural fractions with an acoustic cell embodiment having areflector plate coupled to the wall section opposite said transducer forproducing a standing wave field according to the invention;

FIG. 5E is a photograph illustrating deflections of the fiberspenetrating an ultrasonic field imposing agglomeration and reorientationon the fibers of the fiber suspensions to separate the fibers into twofractions with an acoustic cell embodiment having an absorber platecoupled to a wall section opposite the transducer for producing atraveling wave field according to the invention at the upper limit ofthe laminar flow regime (R≈2000);

FIGS. 5F, 5G, 5H and 5I are photographs illustrating deflections of thefibers penetrating ultrasonic fields (at 20 Watts RMS, 40 Watts RMS, 60Watts RMS and 80 Watts RMS, respectively) to separate the fibers intotwo fractions with an acoustic cell embodiment having an absorber platecoupled to a wall section opposite the transducer for producing atraveling wave field according to the invention in the turbulent flowregime (R≈6000);

FIGS. 6A and 6B are photographs showing an acoustic cell and a dividercell under testing conditions where no acoustic field is applied to theacoustic cell;

FIGS. 6C and 6D are photographs showing the acoustic cell and thedivider cell under testing conditions when the acoustic field is appliedto the acoustic cell, herein a traveling wave field;

FIG. 7A generally illustrates the use of a proposed divider scheme foracoustic fiber separation concept using a traveling wave field, and FIG.7B shows the divider embodiment for acoustic fiber separation using thetraveling wave field, specifically for use under the testing conditionsof FIGS. 6A-6D above;

FIG. 8 generally illustrates the use of a proposed divider scheme foracoustic fiber separation concept using a standing wave field; and

FIG. 9 illustrates the use of the dual concept of acoustic fiberfractionation and clarification.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the present preferredembodiments of the invention, examples of which are illustrated in theaccompanying drawings. FIGS. 1 and 2 illustrate the experimental setupapparatus 10 to study moving fiber suspensions interacting with anultrasonic wave field in a channel flow. The setup can accommodatezero-flow and laminar flow conditions; but the invention is directed toboth laminar and turbulent flow conditions. The maximum allowed fiberconsistency in the described embodiment is 1%. It should be notedhowever that the 1% fiber consistency in this embodiment is a limitationof the pumping equipment used for the subject experimentation, and is inno way representative of limitations on the concept described herein. Infact, consistency is not a limiting factor, it is anticipated thatsatisfactory acoustic fractionation will work at consistencies of 2-3%or more. A suspending fluid (tap or degassed water) is first introducedin the flow system at 12. This procedure provides a means to control theamount of dissolved gas in water when a standing wave field isprescribed; (experimentation as indicated that the standing wave systemmay require degassed water while the traveling wave system does notrequire degassed water). An ultrasonic degassing system 14 with vacuum16 removes gas from the tap water which is stored in a degassed waterreservoir 18 maintained under vacuum 16. The level of gas in the waterintroduced into the apparatus 10 may be measured with a dissolved oxygenmeter 20. Fluid valves 22 and 24 direct the flow of the water in theapparatus 10. The flow direction is reversible. Rinsing water may beintroduced at valve 26 and valves 28 and 30 are used to drain fluid fromthe apparatus 10; an air valve is provided at valve 32.

Fibers are injected and gradually mixed with the suspending fluid. Apump 34, herein a peristaltic pump is shown for moving a continuous feedof pulp slurry through a feed tube 44. Additionally the use of acentrifugal pump to obtain higher flow rates has been found beneficial,but in principal any pump may be used; a pulse dampener 35 may also beused (see FIG. 2). The initial consistency can be increased by simplyadding more fibers with a fiber suspension injector/mixer 36 for fiberaddition and agitation. A constant temperature circulator or heatexchanger 38 is used to control the temperature. An acoustic resonatorassembly 40, including an acoustic cell 42, is coupled to the feed tube44; transparent viewing sections 46 and 48 may be used to observe thefeed stream. On the right side of FIG. 3, the in-line acoustic cell 42(discussed below) is shown mounted vertically to decouple gravitationaland ultrasonic fields.

A computer-controlled function generator 50 and a broadband poweramplifier 50 are used to drive the acoustic cell 42; specifically at aninput for receiving time varying signals coupled to the transducer ofthe cell, as discussed below. The electrical power is limited toapproximately 100 Watts RMS to prevent damage to the transducer of theacoustic cell 42. The function generator 50 is controlled with aconventional PC Computer 54. The output of the power amplifier 52 may bemeasured by the computer 54 using a multimeter 56 interfaced to thecomputer 54. A thermocouple controller 58 is also interfaced to thecomputer 54 for coupling to the acoustic cell 42 to measure thetemperature therein. A rod-mounted, calibrated, 5 mm diameter, sub-MHzP(VDF-TrFE) hydrophone (not shown) is used to evaluate field uniformity,pressure, and power. The wavelength is stabilized against temperaturevariations by using the computer-controlled temperature compensationsystem. A CCD camera 60 and an imaging system 62 are used to recordfiber trajectories in the acoustic cell (see FIGS. 5A-5E below).

Separation methods using standing and traveling wave fields areconsidered here. Focusing on the first case, the acoustic radiationforce acting on a unit length circular cylinder whose axis of symmetryis normal to a plane standing wave field is: ##EQU1## where: a is fiberradius; f(β)= 2 (1-β)/(1+β)!+1! is the inertia factor; β=ρ₀ /ρ₁ is theradio of the suspending medium density to cylinder density; k=2π/λ; andλ is the acoustic wavelength.

The mean energy density is represented as:

    E=(ρ.sub.0 /c).sup.2 /(2ρ.sub.0)

where: ρ₀ is the static pressure; c is the sound velocity; and h is thecylinder center of mass position with respect to a nodal velocity plane.

Equation (1) indicates that F_(SW) is maximum halfway between particlevelocity nodes and anti-nodes and inversely proportional to the acousticwavelength. This specifies that the larger the cylinder radius, thelarger the acoustic force. Providing that β is less than 3, which is thecase for a solid cylinder in a fluid, f(β) is positive and so is F_(SW).Thus, the agglomeration is toward the nearest particle velocityanti-node at stable acoustic equilibrium.

Similarly, A. P. Zhuck, "Radiation Force Acting on a Cylinder Particlein a Sound Field," above, derived an equation for the acoustic forceacting on a cylinder in a traveling wave field which is represented asfollows: ##EQU2##

Turning now to FIG. 3, the acoustic cell 42 is designed to providemaximum flexibility, illustrating the case of a one-dimensional,unbalanced acoustic resonator. The acoustic cell 42 has four identicaland removable modular wall sections 64, 66, 68 and 70 defining a tubularchannel for fluid flow therein. These wall sections are individuallyused to support either a transducer, a reflector plate, an absorberplate, or a viewing port. Herein, wall section 64 supports a reflector72 and wall section 70 supports a transducer 74 opposite the reflector72 for this standing wave system. Wall sections 66 and 68 each supportviewing ports 76 and 78 respectively as opposing glass windows forviewing the fiber trajectories in the acoustic cell therethrough. Thenominal length and width of these components are 100 mm and 20 mm,respectively.

The narrow-band, single-element transducer 74 was designed to resonatenominally at 150 kHz. Its rectangular cross-section Insures that everyfiber penetrating the ultrasonic field is submitted to the same acousticdwell length. It is made of a piezoelectric ceramic material, but anydevice for generating an ultrasonic signal responsive to a time varyinginput will suffice the transducer 74. In the embodiment, slicing of thepiezoelectric material, in the transducer 74, was used to optimizethickness mode vibrations and field uniformity. Considering anapproximate sound velocity in water of 1500 m/s at room temperature, theacoustic wavelength is 10 mm. Since the resonator length(transducer-reflector distance) is 20 mm, λ/2 is 5 mm, and fouragglomeration planes are expected when a standing wave field isproduced. The quarter wavelength is 2.5 mm; it corresponds to thetypical average fiber length for softwood fibers (less for hardwoodfibers). The frequency of ultrasonic output of the transducer 74 isselected in such a way as to prevent contradictory forces acting uponsoftwood fibers in the presence of a standing wave field; otherwise, theprecise frequency may be somewhat arbitrary, especially in the travelingwave field embodiment, discussed below.

FIG. 4A shows a plane traveling wave acoustic field in the acoustic cellembodiment for producing a standing wave field, while FIG. 4B shows aplane standing wave acoustic field in the acoustic cell embodiment forproducing a standing wave field. The traveling wave embodiment: of FIG.4A illustrates the acoustic pressure wave and resulting particlevelocity create agglomeration and reorientation of the fibers beingconcentrated toward the absorber. The standing wave embodiment of FIG.4B illustrates the acoustic pressure wave and resulting particlevelocity create agglomeration planes at half wavelength anti-nodes toinduce deflections of the fibers penetrating the ultrasonic fieldimposing agglomeration and reorientation on the fibers of the fibersuspensions to separate the fibers into the several fractionsillustrated according to the relative sizes of the fibers.

Qualitative experimental results were gathered using rayon fibers andreclaimed wood pulp fibers. Rayon fibers are man-made cellulosic fiberswith known dimensions. In the first set of experiments, rayon fiberssuspended in degassed water were used to investigate acousticagglomeration in a standing wave field under zero-flow condition.Affects of fiber radius and consistency were examined. Selected resultsare reported in FIGS. 5A, 5B and 5C. The transducer's and reflector'spositions are on the left and right sides, respectively. Black areasrefer to fibers because of back illumination. Fiber dimensions andconsistency are as follows; FIG. 5A (length: 3.2 mm; radius; 5.9 μm;0.1% consistency); FIG. 5B (length; 3.2 mm; radius; 5.9 μm; 1%consistency); FIG. 5C (length; 3.2 mm; radius 10.2 μm; 1% consistency).

Four agglomeration planes or layers are observed. At constant fiberradius, the layer thickness increases when the consistency increases(FIGS. 5A and 5B). When the layer thicknesses for the 5.9 μm and 10.2 μmradius fibers at 1% consistency are compared (FIGS. 5B and 5C), oneobserves thinner layers for larger radius fibers. This is a consequenceof the acoustic force depending upon the square of the radius (seeEquation 1); the migration velocity is also increased. As discussedabove, there is no indication that the 1% consistency is an upper limit.In fact a 2-3% consistency appears possible.

In a second set of experiments, the trajectory of rayon fibers suspendedin degassed water was observed at 10 ml/s (FIG. 5D) and 50 ml/s flowrates (approximate figures). Flow velocities in the resonant cavity were2.5 cm/s (R≈500) and 12.5 cm/s (R≈2500), respectively. The consistencywas 1%. While the layers were well formed at R≈500 (laminar flowregime), they were barely seen when the flow rate was increased by afactor of five. Since the acoustic power and/or the acoustic dwelllength could not be increased to enhance fiber deflection, it was notpossible to determine if the acoustic force was hindered by an excessivelevel of turbulence.

In another experiment, flowing reclaimed wood pulp fibers (moreprecisely, OCC or Old Corrugator Container fibers) were observed in atraveling wave field configuration. This is shown in FIG. 5E. The flowrate is 40 ml/s (flow velocity; 10 cm/s; R≈2000). The consistency is0.5%. One can see that the fibers are pushed away from the transducer asthey penetrate the field; they tend to follow a parabolic trajectory.Additional observations obtained using fiber debris (fines) undersimilar flow/acoustic conditions have shown that the debris are barelyaffected by the acoustic field. Hence, the separation of fibers andfiber debris is feasible.

With reference to FIGS. 5F-5I, yet another set of experiments wereperformed in the turbulent flow regime. FIGS. 5F, 5G, 5H and 5I arephotographs illustrating deflections of the fibers penetratingultrasonic fields at 20 Watts RMS, 40 Watts RMS, 60 Watts RMS and 80Watts RMS, respectively. These observations were obtained with softwoodfibers at 0.1% consistency. The flow rate was set constant to 7.21/min(2 gpm--gallon per minute). A centrifugal pump was used. Since theReynold's number is approximately 6000, we have a turbulent flow regime.These four photographs illustrate fiber deflection changes as weincreased the electrical power applied to the transducer (i.e., from 20to 80 W_(RMS)). One can easily see that the fibers are more and moredeflected against the absorber as the power is increased. If the pumpthat employed had been larger, it would have been possible to increasethe flow rate and still observe fiber deflection and separation.

FIGS. 6A and 6B are photographs showing an acoustic cell and a dividercell under testing conditions where no acoustic field is applied to theacoustic cell using Rayon fibers at a consistency of 0.25%. FIGS. 6C and6D are photographs showing the acoustic cell and the divider cell underthe same testing conditions when the acoustic field is now applied tothe acoustic cell, herein providing a traveling ultrasonic wave fieldthrough the cell. Note particularly the separation along the flowdirection of the stream into a high consistency stream to the left ofthe divider wall, and a clean stream to the right of the divider wall.

Qualitative results presented in FIG. 5A through FIG. 5D support thestanding wave field acoustic separation scheme illustrated in FIG. 8;wherein a mechanical divider connected to the down stream side of theacoustic cell divides a plurality of agglomeration fraction streams asthe concentrated paper fiber stock stream of relatively larger fibersseparated out from a plurality of fraction streams substantially free offibers as the clean stream of the relatively smaller fibers. Largeradius fibers are deflected toward anti-nodal particle velocity planesand collected appropriately (see FIG. 4B above). Fiber defection (andseparation) can be controlled by varying the flow rate, the acousticpower, the acoustic dwell (length of tranducer 74) and/or the frequency(assuming a broadband transducer 74). While the resonator length wouldbe relatively small (few centimeters), its width would need to besufficiently large to accommodate an acceptable throughput. Practicalconsistency would be 0.5 to 2%. Pressurization may be advantageous toeliminate the need for degassed water. Such a pressurized system mayalso provide additional control by allowing the control of the dischargeflow rates as in pressure screen systems.

A separation scheme similar to the one discussed can be envisioned for atraveling wave field configuration. Here, qualitative results presentedin FIG. 5E supports the traveling wave field acoustic separation schemeillustrating a deflection trajectory of fibers penetrating an ultrasonicfield imposing agglomeration and reorientation on the fibers of thefiber suspensions to separate the fibers into two fractions. Asillustrated in FIGS. 7A and 7B, here mechanical dividers connected tothe down stream side of the acoustic cell divides an agglomerationfraction stream as the concentrated paper fiber stock stream ofrelatively larger fibers separated out from a fraction stream of therelatively smaller fibers. FIG. 7B shows the divider embodiment foracoustic fiber separation using the traveling wave field, specificallyfor use under the testing conditions of FIGS. 6A-6D above. Again, fiberdefection can be controlled by varying the flow rate, the acousticpower, the acoustic dwell and/or the frequency; the transducer-absorberseparation distance can also be adjusted depending on the intendedapplication. Moreover, the position of the mechanical divider can beadjusted for additional control over the separation of the concentratedpaper fiber stock stream.

With reference to FIG. 9, one might take advantage of a higher frequencyultrasonic field to deflect already separated short fibers, and thusprovide water for dilution. This is the basis for a dual concept ofacoustic fiber fractionation and clarification as shown. This two-stagescheme uses the acoustic fractionator cell 42 and a like cell at higherfrequencies, acoustic clarifier to get high short fiber consistency 82and high long-fiber consistency 84 stocks, and dilution water 86.

The use of a high frequency ultrasonic field may also be used toseparate fiber debris or fines from contaminants such as ink particleson the basis that the density would be different in both cases. A moreadvanced fractionation concept using two traveling wave fieldspropagating in opposite directions may also be employed for generatingwaves in the channel of an acoustic cell. Instead of attempting togenerate a standing wave field, the two fields (identical or differentfrequencies) also may be used to position a deflection plane anywherebetween the two transmitting transducers providing other fractionationpossibilities. Of course, it will be appreciated by those skilled in theart that modifications to the foregoing preferred embodiments may bemade in various aspects. The present invention is set forth withparticularity in the appended claims. It is deemed that the spirit andscope of that invention encompasses such modifications and alterationsto the preferred embodiment as would be apparent to one of ordinaryskill in the art and familiar with the teachings of the presentapplication.

What is claimed is:
 1. An acoustic cell for subjecting a suspension offibers to deflection forces, the fibers being of differing relativesizes, the acoustic cell comprising:an elongated tubular element havingat least one inlet channel for receiving at least one continuouslymoving stream of a fiber suspension in a directed flow, said tubularelement having more than one downstream outlet; an input for receivingtime varying signals; and at least one transducer coupled to said inputand coupled to said tubular element for generating at least oneultrasonic field which is transverse to said flow and responsive to thetime varying signals received at said input, said transducer effectivefor generating at least one ultrasonic field transverse to said flow ofsufficient energy to induce deflections of the fibers penetrating saidat least one ultrasonic field, said at least one ultrasonic fieldeffective for imposing agglomeration and reorientation on the fibers ofthe fiber suspension along a trajectory to separate the fibers into atleast two fractions according to the relative sizes of the fibers, saidat least one continuously moving stream flowing through said tubularelement, and said at least one ultrasonic field separating the fibers ofthe fiber suspensions into at least two fractions corresponding to atleast two outlet streams of fibers at the downstream outlets of saidtubular element.
 2. An acoustic cell as recited in claim 1 wherein saidtubular element comprises four wall sections.
 3. An acoustic cell asrecited in claim 2 wherein each of said wall sections compriserectangular removable modular sections.
 4. An acoustic cell as recitedin claim 2 wherein said transducer comprises piezoelectric ceramicmaterial.
 5. An acoustic cell as recited in claim 4 wherein saidtransducer is designed to resonate at approximately 150 kHz.
 6. Anacoustic cell as recited in claim 4 wherein said input comprises anelectrical input for receiving time varying electrical signalselectrically coupling the electrical signals to said piezoelectricceramic material of said transducer for generating the ultrasonic fieldresponsive to the time varying signals received at said input.
 7. Anacoustic cell as recited in claim 6 wherein said transducer is coupledto a first one of said wall sections opposite a second one of said wallsections for generating the ultrasonic field to induce the deflectionson the fibers of the fiber suspensions between the opposing first andsecond wall sections.
 8. An acoustic cell as recited in claim 7comprising a reflector plate coupled to said second wall sectionopposite said transducer for producing a standing wave field between theopposing first and second wall sections.
 9. An acoustic cell as recitedin claim 7 comprising an absorber plate coupled to said second wallsection opposite said transducer for producing a traveling wave fieldbetween the opposing first and second wall sections.
 10. An acousticcell as recited in claim 7 comprising a first viewing port on a thirdone of said wall sections, said third wall section being positionedintermediate said first wall section and said second wall section, forviewing the agglomeration and reorientation of the fibers in the fibersuspension between the opposing first and second wall sections.
 11. Anacoustic cell as recited in claim 10 comprising a second viewing port ona fourth one of said wall sections, said fourth wall section beingpositioned intermediate said first wall section and said second wallsection and opposite said third wall section, for viewing theagglomeration and reorientation of the fibers in the fiber suspensionbetween the opposing first and second wall sections through the opposingthird and fourth wall sections.
 12. An acoustic cell as recited in claim11 wherein said first viewing port and said second viewing port eachcomprise a glass window.
 13. An acoustic fractionator for separating atleast one suspension of fibers into plural fractions according to therelative fiber sizes of differing fibers, the acoustic fractionatorcomprising;a feed tube which conveys a continuous feed stream of fibersuspension; an acoustic cell connected to said feed tube which subjectsthe suspension of fibers to deflection forces; at least one pump whichmoves the stream of fiber suspension through the feed tube and acousticcell, the acoustic cell comprising an elongated tubular element inletchannel which receives the at least one continuously moving stream offiber suspension in a directed flow, said tubular element having morethan one downstream outlet, an input for receiving time varying signals,and at least one transducer coupled to said input and coupled to saidtubular element for generating at least one ultrasonic field which istransverse to said flow, and responsive to the time varying signalsreceived at said input, said transducer effective for generating atleast one ultrasonic field transverse to said flow of sufficient energyto induce deflections of the fibers penetrating said ultrasonic field,said at least one ultrasonic field effective for imposing agglomerationand reorientation on the fibers of the fiber suspension along atrajectory to separate the fibers into continuously moving streams of atleast two fractions according to the relative sizes of the fibers, saidfiber suspension flowing into more than one fraction corresponding tosaid more than one downstream outlet; and a divider connected to saidacoustic cell for dividing the continuously moving stream flowingthrough said tubular element into the more than one isolated fractionsaccording to the relative sizes of the fibers at said downstream outletof said tubular element.
 14. An acoustic fractionator as recited inclaim 13 wherein said feed tube conveys paper mill effluents containingpulp slurry recyclable through separation of large radius fibers andsmall radius fibers into a concentrated paper fiber stock of therelatively larger fibers for paper making and a clean stream of therelatively smaller fibers from the stream of dilute fiber suspension.15. An acoustic fractionator as recited in claim 14 wherein said pump isa peristaltic pump for moving a continuous feed of pulp slurry throughsaid feed tube.
 16. An acoustic fractionator as recited in claim 14wherein said divider comprises a mechanical divider connected to thedown stream side of said acoustic cell for dividing an agglomerationfraction stream as the concentrated paper fiber stock stream ofrelatively larger fibers separated out from a fraction stream of therelatively smaller fibers.
 17. An acoustic fractionator as recited inclaim 16 wherein said tubular element comprises four wall sections, saidtransducer comprises piezoelectric ceramic material, said inputcomprises an electrical input for receiving time varying electricalsignals electrically coupling the electrical signals to saidpiezoelectric ceramic material of said transducer for generating theultrasonic field responsive to the time varying signals received at saidinput, said transducer being coupled to a first one of said wallsections opposite a second one of said wall sections for generating theultrasonic field to induce the deflections on the fibers of the fibersuspensions between the opposing first and second wall sections, saidacoustic cell further comprising an absorber plate coupled to saidsecond wall section opposite said transducer for producing a travelingwave field between the opposing first and second wall sections.
 18. Anacoustic fractionator as recited in claim 17 wherein said dividercomprises a mechanical divider connected to the down stream side of saidacoustic cell for dividing an agglomeration fraction stream as theconcentrated paper fiber stock stream of relatively larger fibersseparated out from a fraction stream substantially free of fibers as theclean stream of the relatively smaller fibers.
 19. An acousticfractionator as recited in claim 16 wherein said tubular elementcomprises four wall sections, said transducer comprises piezoelectricceramic material, said input comprises an electrical input for receivingtime varying electrical signals electrically coupling the electricalsignals to said piezoelectric ceramic material of said transducer forgenerating the ultrasonic field responsive to the time varying signalsreceived at said input, said transducer being coupled to a first one ofsaid wall sections opposite a second one of said wall sections forgenerating the ultrasonic field to induce the deflections on the fibersof the fiber suspensions between the opposing first and second wallsections, said acoustic cell further comprising a reflector platecoupled to said second wall section opposite said transducer forproducing a standing wave field between the opposing first and secondwall sections.
 20. An acoustic fractionator as recited in claim 19wherein said divider comprises a mechanical divider connected to thedown stream side of said acoustic cell for dividing a plurality ofagglomeration fraction streams as the concentrated paper fiber stockstream of relatively larger fibers separated out from a plurality offraction streams substantially free of fibers as the clean stream of therelatively smaller fibers.
 21. A method of separating at least onesuspension into more than one fraction according to the relative fibersizes of differing fibers, comprising the steps of:conveying acontinuous feed stream of fiber suspension; transferring the feed streamthrough an acoustic cell having a tubular element having more than onedownstream outlet, said acoustic cell having at least one ultrasonictransducer transverse to said flow for subjecting the least onesuspension of fibers to deflection forces in the tubular element of theacoustic cell; generating at least one ultrasonic field with theultrasonic transducer upon the channel flow transverse to said flow ofsufficient energy to induce deflections of the fibers penetrating saidat least one ultrasonic field in the acoustic cell, said at least oneultrasonic field effective for imposing agglomeration and reorientationon the fibers of the fiber suspension along a trajectory to separate thefibers into more than one continuously moving stream of more than onefraction according to the relative sizes of the fibers, the ultrasonicfield separating the fibers of the fiber suspension into more than onefraction; and dividing said separated more than one fraction of the atleast one fiber suspension at the downstream side of the acoustic cellwith the outlets.